Optical communication networks are rapidly being deployed to transmit both voice and data. Because the carrier frequencies used in optical fiber are orders of magnitude higher than used in any other communications medium, the bandwidth (i.e., data rate) afforded by such systems is inherently greater than twisted pair copper cables or coaxial cable.
Nevertheless, the majority of optical networks use electronic devices for signaling because electronic routing and signal processing devices are well developed. In some cases, sophisticated protocols requiring complex electronic logic are used.
Electronic processing requires converting an optical signal to the electronic domain prior to processing, then converting it back to the photonic domain for transmission. These conversions are time-consuming and a major bottleneck in optical networks.
All-optical switching technology, in which one optical signal is switched on or off upon signaling by an optical control beam, allows optical switching to be done without conversion, eliminating the optical-electrical-optical conversion bottleneck. Examples of such devices include Nonlinear Optical Loop Mirrors and Semiconductor Optical Amplifier-based devices.
Another type of all-optical switch is a Quantum Dot Saturable Absorber switch, which is disclosed in U.S. patent application Ser. No. 09/737,470, filed Dec. 18, 2000 and entitled “Optical Switch Having A Saturable Absorber”, the disclosure of which is herein incorporated by reference. The Quantum Dot Saturable Absorber switch operates by controlling the optical properties of a quantum dot material such as optical absorption or index of refraction with an external beam of light. It is much faster than conventional electronic devices, has low losses, and is well-suited for integration into larger-scale devices and systems.
A widely-used technique for transmitting and routing signals in optical communication networks is Time Division Multiplexing (“TDM”). This consists of dividing up a frame, or window in time, into multiple evenly-spaced time slots, and synchronously inserting a single bit of data from lower-bandwidth sources into a higher-bandwidth multiplexed stream. Recovering the signal consists of a demultiplexing operation in which the high-bandwidth stream is split into the individual low-bandwidth sources. The inherent simplicity of electronic TDM renders the processing logic and the devices required to be fairly straightforward and more easily implemented than complex protocols.
Optical time division multiplexing (“OTDM”) is performed entirely optically without electronic conversion. Prior art OTDM implementations have encountered technological constraints that have prevented its implementation. For example, U.S. Pat. No. 5,493,433, entitled “Terahertz optical asymmetric demultiplexer” discloses an Optical Time Division Multiplexor system that includes a fiber loop that contains a nonlinear optical element placed asymmetrically within the loop. The entire device functions as an optical AND gate that allows an optical pulse to pass through when in the appropriate time slot. The device operates by first splitting the input signal pulses into two beams which are coupled to the optical fiber loop but travel in opposite directions around the loop. An optical control pulse is timed to alter the index of refraction of the nonlinear optical element so that a phase difference is generated between the counter-propagating optical signal pulses. When the optical signal pulses traverse the fiber loop they are coupled back together. If the pulses are in phase, constructive interference occurs and the pulses can exit the device. However, if the optical pulses are out of phase, deconstructive interference occurs and the optical signal cannot pass.
Optical loop type devices, such as disclosed in U.S. Pat. No. 5,493,433, are fairly fast (on the order of picoseconds) but are very power hungry and large. Therefore, these devices requires extremely expensive components and are difficult to integrate into more complex systems because of their large size. In addition, these types of devices are very sensitive to environmental factors such as temperature, because the refractive index of the fibers are related to temperature and because the length of fiber within the loop mirror is so long. Therefore, a small temperature change greatly alters the performance of the device.
Based on the foregoing, there is a need for an improved optical time division multiplexing/demultiplexing system.